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New chikungunya strain spreads from mosquito to mosquito

September 9th, 2010, 04:42 PM

New chikungunya strain spreads from mosquito to mosquito

9 September 2010 | EN

[NEW DELHI] Scientists who discovered a deadly strain of chikungunya virus in India also found that the virus can spread from mosquito to mosquito, bypassing the human transmission stage. This could lead to longer-lasting outbreaks affecting more people.

Scientists at the Rajiv Gandhi Centre for Biotechnology (RGCB) in Thiruvanathapuram, Kerala said this highlighted the need for stricter surveillance and mosquito control measures.

Chikungunya virus, spread mainly by Aedes albopictus mosquitoes and Aedes aegypti that also spread the dengue virus, has caused severe outbreaks across Asia, Africa and Latin America during this decade. Typical symptoms include fever, headache, severe joint pain and nausea.

The scientists analysed a pool of new, short-lived virus strains recorded during a chikungunya outbreak in 2009 in the southern state of Kerala.

They reported in the Virology Journal last month (13 August) that some strains carried the A226V mutation first recorded in the Indian Ocean Island of Reunion during an unprecedented outbreak in 2005-2006 when some 26,000 cases were registered — almost a third of the Island’s population. The infection, previously considered non-fatal, also led to abnormally high death rates during that outbreak.

The re-emergence of chikungunya virus in Reunion puzzled scientists as it was not spread by the usual Aedes aegypti mosquito, but by another mosquito Aedes alpictus that is also common in Asia.

In 2007 scientists reported that a single genetic change, the A226V mutation, altered one of the virus envelope proteins E1, which help the virus enter mammals, including humans, and also adapt to new mosquitoes.

Now, RGCB researcher Easwaran Sreekumar and his team have identified a strain with a mutation in a second envelope protein E2, which helps the virus enter mosquitoes. They also found that the virus can transmit from mother to daughter mosquitoes, bypassing the human transmission stage, which makes it spread quickly through the mosquito population, potentially causing bigger outbreaks.

"It does not need a fresh human host and so can persist for a longer time in a community," Sreekumar said.

If other studies confirm this finding, "we can anticipate persistence of Chikungunya in the community for a longer time till a very large proportion of the people get infected and develop immunity," he added.

Persistence will also enable the virus to accumulate more genetic changes, leading to a more virulent version. "In such situations, as we have now in India, it is essential that we regularly monitor the emergence of new mutant lineages that might have altered characters," Sreekumar said.

Chikungunya virus (CHIKV), an arthritogenic alphavirus, is transmitted to humans by infected Aedes (Ae.) aegypti and Ae.albopictus mosquitoes. In the study, reverse-transcription PCR (RT PCR) and virus isolation detected CHIKV in patient samples and also in adult Ae.albopictus mosquitoes that was derived from larvae collected during a chikungunya (CHIK) outbreak in Kerala in 2009.

The CHIKV strains involved in the outbreak were the East, Central and South African (ECSA) genotype that had the E1 A226V mutation. The viral strains from the mosquitoes and CHIK patients from the same area showed a close relationship based on phylogenetic analysis.

This novel, non-conservative mutation, L210Q, consistently present in both human and mosquito-derived samples studied, was within the region of the E2 protein (amino acids E2 200-220) that determines mosquito cell infectivity in many alpha viruses.

Our results show the involvement of Ae. albopictus in this outbreak in Kerala and appearance of CHIKV with novel genetic changes.

Detection of virus in adult mosquitoes, emerged in the laboratory from larvae, also points to the possibility of transovarial transmission (TOT) of mutant CHIKV strains in mosquitoes.

“Addressing chronic disease is an issue of human rights – that must be our call to arms"
Richard Horton, Editor-in-Chief The Lancet